1. Field of the Invention
This invention relates to zoom lenses including wide angle regions for photography, and more particularly to two-group type zoom lenses of small size with an aspheric surface.
2. Description of the Prior Art
As the zoom lenses include wide angle regions for photography, wide spread use is made of the zoom type comprising two lens groups of which the first, counting from the front, is of negative refractive power, and the second is of positive refractive power with the axial air separation between both lens groups made variable to effect zooming. This is because the negative lens group-preceded or so-called two-group type zoom lens is suited for aberration correction in the wide angle region.
However, even with the two-group type zoom lens, when great importance is placed on the imaging performance in the wide angle regions, and particularly when the zooming range is extended toward shorter focal lengths with the resultant prominently intensified distortion well corrected, the number of lens elements in the first group must be increased, or otherwise such correction would be very difficult to perform. The use of the increased number of lens elements in facilitating the aberration correction, however, necessarily results in a complicated lens construction of the first lens group which in turn causes the bulk and size of the lens system to be increased and the handling to become difficult. The production cost of the objective also increases.
The known practice in the art is to employ an aspherical lens constituting a part of the lens system for assisting in the stabilization of the aberrations throughout the extended zooming range. The lens, to which the aspherical surface is applied, is formed with the use of a plastic material, contributing to a decrease in the production cost of the objective. These ideas are proposed in U.S. Pat. Nos. 4,159,865 and 3,912,379 and Japanese Laid Open Patent Application No. Sho 54-139752.